2 Answers
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An academic reason would be that indeed Hebrew (and other related languages) don't need vowels for disambiguation as much as, say, English. Most Hebrew words are built out of triliteral consonantal roots, so that words with the same consonants are (usually) related, differing only in how they're inflected for different parts of speech, number, tense and so forth. Contrast with English, where the vowels play a much more important part in the etymology of words, and taking them out indeed causes a great deal of ambiguity (which is why "disemvoweling" is effective for trollish comments).

Also, classical Hebrew has a CV(C) syllable structure, meaning that a word can't begin with a vowel sound. (Most Jews don't pronounce the letter א, but properly speaking, it is supposed to be a glottal stop.) By contrast, an English syllable can begin with anywhere from zero to three consonants, so there's much greater potential for confusion.

A more classical Jewish approach, as YDK pointed out, is that this very ambiguity allows us to derive multiple layers of meaning from the written text. In the case of G-d's name mentioned in the original (linked and closed) question, it can in fact be written with many different sets of vowel signs, each of which symbolizes some particular way in which He relates to us and we to Him. In other cases, details of Jewish law or thought are arrived at by contrasting the way in which a word is actually vocalized (called in the Talmud mikra) with other possible ways that the same series of consonants could be pronounced (called masores).

But what with foreign words like for example names of real people? How can a jew know how to pronounce name of people he never heard of? For example name of germany chancellor Angela Merkel would be written in hebrew as מרכל. Possible pronounciations are for example Markel, Markal, Marakal or even Marchal (because dagesh marks are commonly not used). There is quite large space for ambiguity. Such writing system seems very impractical for me.
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truthseekerOct 1 '11 at 19:31

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@truthseeker, no reason you can't put in the vowel markings in such cases. (Even for native Hebrew words, this is sometimes done for disambiguation.) But we're talking here primarily about the Hebrew alphabet as it is used to write the Bible and other sacred writings; I guarantee you Ms. Merkel's name doesn't appear in them. (And after all, even fully alphabetic languages sometimes have the same problem too. Would you know how to pronounce "Recep Tayyip Erdogan" properly based on the spelling, if you don't know Turkish?)
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AlexOct 2 '11 at 1:47

Rabbeinu Bachye on "lo sechanem" (Vaeschanan, 7:2) gives multiple ways of reading the prohibition based on fiddling with the vowels. He gives this flexibility for multiple versions as the reason for the Torah not including vowels. See R. Bachye also Behaalos'cha 11:15.